The present invention relates to a friction-drive type drive unit using a piezoelectric element.
There is publicly known a friction-drive type drive unit in which a shaft-like vibrating member is reciprocatively displaced asymmetrically in its axial direction by a piezoelectric element so that a frictional engagement member frictionally engaged with the vibrating member is slidingly displaced relative to the vibrating member (which can be done by moving the frictional engagement member or by moving the vibrating member).
Such a friction-drive type drive unit involves a drive circuit for applying a periodically varying voltage to the piezoelectric element. Utilized therefor among others is a drive circuit which applies a rectangular-wave voltage to the piezoelectric element by applying a voltage to the electrodes of the piezoelectric element or grounding those electrodes with the use of a semiconductor switching element.
For example, JP2001-211669A discloses a full bridge circuit for connecting both electrodes of a piezoelectric element alternately to power supply while grounding the other electrode so that a polarity applied to the piezoelectric element is inverted.
In such a friction-drive type drive unit for drive with a rectangular-wave voltage as shown above, the frictional engagement member keeps a constant amount of sliding displacement per pulse of the rectangular-wave voltage, thus being incapable of obtaining any displacements lower than the constant amount.
Changing the duty ratio of the rectangular-wave voltage makes it possible to reduce the sliding displacement amount of the frictional engagement member per pulse. However, when the duty ratio of the rectangular-wave voltage is changed to reduce the sliding displacement amount of the frictional engagement member, the moving speed of the frictional engagement member that is moved in frictional engagement with the vibrating member would be increased, causing the driving torque to decrease, as a problem.
Also, in the friction-drive type drive unit, keeping the frictional engagement member continuously displaced within a narrow range would lead to changes in driving characteristics due to wear or the like, as a further problem.